1. Field of the Invention
The invention relates to a method for hot dip coating a flat steel product manufactured from a stainless steel which contains more than 5 wt. %, in particular at least 10.5 wt. %, Cr with a protective metallic coating to protect it against corrosion. “Flat steel products” here means steel strips or steel sheets.
2. Description of Related Art
Steels of the type in question with a high chromium content above 5 wt. % and typically up to 30 wt. % characteristically have a particularly good chemical resistance and high corrosion resistance. These product properties are based on the formation of a stable chromium oxide layer which passivates the steel surface effectively against external influences even at high temperatures. This means that steel goods with a chromium proportion of >10.5 wt. % are also termed rust, heat and acid resistant, or stainless steels for short. Further alloy elements such as nickel or molybdenum can help this passivation.
Despite these excellent specific material properties in relation to environmental influences, the use of chromium-alloyed steels for particularly stressed components can make it technically necessary and/or economically sensible to apply an additional protective coat.
The chemical passivity of the covering layer of chromium oxide is a problem here. This layer hinders both the wetting and the adhesion reaction when coating with a metallic coating.
Coating steels with at least 5 wt. % Cr thus presents a particular challenge.
It is known from AT 392089 B that stainless steels can be galvanised on one side and on both sides electrolytically in a continuous strip process. However, this method is comparatively expensive and has therefore not been implemented in practice.
A cost-efficient alternative to electrolytic coating is the continuous hot dip coating of steel strips. In this method, after recrystallising annealing has been carried out on a steel strip in a continuous furnace, it is submerged for a short period into a metallic molten bath which is typically based on zinc, aluminium or alloys thereof.
The hot dip coating of alloyed steels requires particular care, since with these steels, during the annealing phase alloy components which have a particular affinity for oxygen can selectively oxidise on the surface of the steel. If the selective oxidisation takes place externally, i.e. with the oxygen from the atmosphere, problems with wetting and a lack of adhesion are to be expected.
For high strength/maximum strength multiphase steels which have a comparatively low, typically 0.3-2.0% Cr alloy proportion, a method described in EP 2 010 690 B1 is proven in which the respective flat steel product is heated in a first work step in a reducing atmosphere with an H2 content of at least 2% by volume to 8% by volume to a temperature of >750° C. to 850° C. and in which the surface, which mainly consists of pure iron, is then transformed into an iron oxide layer by heat treatment of the flat steel product at a temperature of >750° C.-850° C. lasting 1 to 10 seconds in a reaction chamber integrated into a continuous furnace with an oxidising atmosphere with an O2 content of 0.01% by volume to 1% by volume and in which the flat steel product is then annealed in a reducing atmosphere with an H2 content of 2% by volume to 8% by volume by heating to a maximum of 900° C. over a time period which is so much longer than the duration of the heat treatment carried out to form the iron oxide layer that the previously formed iron oxide layer is reduced to pure iron at least on its surface. The flat steel product pre-treated in this way can be hot dip coated with the metallic coating in a warmed state in a molten bath which contains overall at least 85 wt. % zinc and/or aluminium.
A flat steel product hot dip coated with aluminium for exhaust systems is further known from EP 2 184 376 A1. However, this document does not suggest how the hot dip coating can be carried out in practice. The possibility of pre-coating with iron is suggested, which would make aluminium dip coating considerably easier but is more expensive.
In principle, two types of method are known for the hot dip coating of steels with more than 5 wt. % Cr, in particular more than 10 wt. % Cr which each assume that the steel strip to be coated can be prepared using an annealing treatment such that an optimal coating is achieved.
The first type of method provides for annealing under drastically reducing atmosphere.
A variant of this type of method is described in U.S. Pat. No. 4,675,214 (EP 0 246 418 B1), U.S. Pat. No. 5,066,549 and U.S. Pat. No. 4,883,723. This variant assumes that the flat steel product to be coated is heated in a non-oxidative atmosphere and then held at more than 677° C. in a drastically reducing atmosphere with over 95% by volume H2/N2 for steels with 6.0-14.5 wt. % Cr. The coating is then carried out in an aluminium or aluminium/silicon molten bath.
An additional variant of the first type of method is known from U.S. Pat. No. 5,023,113. This variant is based on flat steel products with a chromium content >10 wt. %. These flat steel products are heated to 650° C. with no free oxygen and then held at 845-955° C. under an atmosphere which contains >95% by volume H2/N2. In addition to this, in the nozzle through which the steel strip is carried from the furnace to the molten bath, should have atmosphere >97% by volume H2/N2 with a dew point of <−29° C.
A third variant of the first type of method is known from U.S. Pat. No. 5,591,531. According to this variant, steel strips with up to 30 wt. % Cr are subject to batch annealing which creates a surface layer that is rich in iron. The actual annealing then takes place in accordance with one of the two above mentioned variants of the first type of method.
The method known from EP 0 467 749 B1 (DE 691 04 789 T2) avoids these annealing conditions by preheating to temperatures of less than 500° C. under a non-oxidising atmosphere which may therefore contain <3% by volume O2. It is then further heated to a holding temperature of less than 950° C. in a non-oxidising, non-reactive N2 or H2/N2 atmosphere with a dew point below −40° C. An Al or AlSi melt is also used for the hot dip coating.
The second known type of method is based on the use of the oxidation/reduction technique (“pre-oxidation”).
A first variant of this second type of method is described in JP 3111546 A. In accordance with this known method, a steel strip alloyed with 10.0%-25.0 wt. % Cr is oxidised in a directly fuelled pre-heater at temperatures of 400-600° C. The FeO layer created in this way is then reduced during a holding phase at 700-950° C. The steel strip which has been treated in this way is then subject to hot dip aluminising.
According to JP 5311380 A in accordance with a second variant of the second type of method a steel strip containing 10.0%-25.0 wt. % Cr is hot dip aluminised in a similar manner. In this way, the pre-oxidation also takes place during heating up directly to a temperature between 550-750° C. by regulating the X value to 0.9-1.5. The reduction of the FeO layer then takes place under a reducing atmosphere at a holding temperature which is around 800° C. or reaches up to a maximum of 1050° C.
The first type of method can only be carried out at high cost in everyday work using hot dip coating equipment designed for conventionally alloyed steel. The necessary high annealing temperatures and the high consumption of H2 result in considerably increased operating costs. Commercial practice also shows that a dew point <−40° C. cannot be reliably maintained in the holding zone of the continuous furnace.
The variants belonging to the second type of method could be achieved considerably easier as part of a commercial hot dip coating process. However, here too operating practice shows that the problems with wetting in flat steel products made of steels with high Cr contents cannot be reliably avoided. Particularly the low pre-oxidation temperatures given in JP 3111546 A prove to be particularly critical in the operation conditions used in practice.
A further disadvantage of the type of method described above is that this method only relates to hot dip aluminising.
Against this background, the object of the invention was to provide a method which allows flat steel products provided for applications particularly subject to corrosion, containing more than 5.0 wt. % chromium, to be provided with hot dip coating in a manner which is cost-effective and environmentally friendly.